The present invention relates to an exposure apparatus using a reflective optical element.
Display elements used for a television or a personal computer, such as liquid crystal displays (LCDs), have been required, in order to provide a larger screen. LCDs are manufactured by a photolithography (baking) technique to form a transparent thin-film electrode on a glass substrate. The exposure apparatus (liquid crystal exposure apparatus) that manufactures the LCD has a projection optical system, by which a mask and the glass substrate are scanned at a slit-like region to form a pattern of the transparent thin-film electrode.
With more displays having a larger screen (i.e., a larger glass substrate), more liquid crystal exposure apparatuses have been provided with a larger region in which exposure light is irradiated. However, an expansion of the exposure light irradiation region causes a decreased illuminance, preventing the throughput from being improved.
To solve this, a liquid crystal exposure apparatus that uses the projection optical system (mirror projection optical system) constituted by a reflective mirror (reflective optical element) has been proposed.
Since the mirror projection optical system causes no chromatic aberration in principle, when a mercury lamp is used as a light source, all bright line spectra of the mercury lamp can be simultaneously used as exposure energy.
Thus, broadening the wavelength band of exposure light (i.e., the wavelength band may be broadened to a line I, a line g, a line h, and a line j at a shorter wavelength side) can provide high illuminance.
On the other hand, alignment light used for the position adjustment (alignment) between the mask of the exposure apparatus and the glass substrate requires a reflective mirror that has a high reflectivity in a different wavelength from that of exposure light, and that can provide a desired phase difference between a reflected S-polarized light component and a reflected P-polarized light component. Furthermore, when several types of alignment lights are used, the above optical characteristic is required in all wavelength bands used for the alignment.
For this reason, the reflective mirror constituting an optical system of the liquid crystal exposure apparatus needs to have a desired reflection characteristic in a wavelength region of exposure light, and the desired reflection characteristic and the phase difference characteristic in a wavelength region of alignment light.
Conventionally, the reflective mirror that controls the phase difference includes a multilayer film mirror structured so that a metal film having a high reflectivity, such as aluminum (Al), copper (Cu) or silver (Ag), is formed on a surface of the glass substrate, and the metal film has thereon a protection film that protects corrosion and an enhanced reflection film that enhances reflectivity. The multilayer film mirror, as described above, is disclosed in Japanese Patent Unexamined Publication No. H06-138310 and Japanese Patent Unexamined Publication No. 2006-227099.
As a reflective mirror in which all layers consist of a dielectric material, a multilayer film mirror, in which a highly-refractive material and a lowly-refractive material are alternately laminated, is disclosed in Japanese Patent Unexamined Publication No. H05-215915.
Although a conventional reflective mirror composed of a metal and a dielectric material shows a stable phase difference characteristic (characteristic having a smaller manufacturing error), the conventional reflective mirror has a limited reflectivity characteristic and an abrasion resistance characteristic. For example, an absorption characteristic of the reflective mirror required for the exposure apparatus needs to be very small. The reason for this is that the exposure apparatus requires a clear reflected image, and has a deteriorated resolution, due to influence from heat that is generated. In other words, the reflective mirror requires almost 100% reflectivity. However, the use of a metal film finds a difficulty in achieving almost 100% reflectivity.
The conventional reflective mirror also requires a high abrasion resistance characteristic, because the reflective mirror must be handled with great care, so as to prevent any flaws from being generated on the surface of the reflective mirror, which causes difficulties in maintenance.
In the case of a full dielectric material mirror, in which all layers having superior absorption characteristics and abrasion resistance characteristics consist of a dielectric material, a ¼ wavelength film thickness multilayer group, for example, has a maximum reflectivity and reflection bandwidth, and the band can provide a phase difference characteristic having a substantially-fixed value. Furthermore, the full dielectric material mirror can broaden a width of a high reflectance region by laminating the ¼ wavelength film thickness multilayer groups having different period lengths.
However, the phase difference characteristic obtained by laminating the ¼ wavelength film thickness multilayer groups as described above has a so-called steep ripple, and has no stable characteristic in a wide band. Specifically, although a high reflectance band is broadened, a stable phase difference characteristic (characteristic having a small manufacturing error) is not obtained in this case.
In the current situation, no reflective mirror exists, as required by the exposure apparatus, for example, that has a superior abrasion resistance characteristic, that has a wide band and a high reflectivity, and that has a mild change in the characteristic to a wavelength of a reflection phase difference of S-polarized light and P-polarized light.